1. Field of the Invention
The present invention relates to a step-slide type cutter knife with automatic lock mechanism and more particularly to the cutter knife having a mechanism for locking a slider, movably inserted into a slide groove of a cutter main body and connected with a cutter blade, when the cutter knife is in use.
2. Description of the Related Arts
A mechanism for locking the slider of the cutter knife, in which a part of a spring held by the slider engages engaging notches formed longitudinally in regular intervals in the wall formed on a side surface of a slide groove in the cutter main body, is disclosed in Japanese Utility Model Publication No 2-20991.
According to the cutter knife disclosed in this Publication, the slider comprises a slider main body; and an operating member movable with respect to the slider main body in the moving direction of the cutter blade. The slider main body holds the center of a V-shaped leaf spring or a torsion spring. The slider is locked when both free ends of the spring engage with engaging notches of the cutter main body. A release strip is disposed on the operating member between both free ends of the spring so that it can move with the operating member. The release strip disengages one of the free ends of the spring from the engaging notch when the operating member is moved forward or backward with respect to the slider main body.
In this construction, when the slider is operated, the operating member moves in the moving direction of the blade with respect to the slider main body. As a result, the forward free end of the spring disengages from the engaging notch, thus allowing the movement of the cutter blade. When an operator's hand is released from the slider, each free end engages the engaging notch. As a result, the slider is locked with the cutter main body.
The construction of the locking mechanism of this cutter knife has a disadvantage that force applied to the cutter blade acts on the spring as a bending load during the use thereof. Accordingly, when a great force is applied to the cutter blade, the spring is deformed, which may damage the locking mechanism. Therefore, it is difficult to apply the locking mechanism to a large cutter knife.
There is proposed in Japanese Utility Model Publication No. 61-42522 a cutter knife comprising a locking mechanism for preventing force from being applied from the cutter blade to the spring. According to the cutter knife, the slider comprises the slider main body and the operating member similarly to the above-described cutter knife. A second spring for normally keeping the operating member at an intermediate position of its movable range is provided separately from a locking spring which engages the engaging notch of the cutter main body. The operating member has a locking section for supporting the engaging portion of the locking spring from the direction opposite to the disengaging direction of the engaging portion so that the engaging portion does not disengage from the engaging notch when the operating member is at the intermediate position as a result of the release of the operator's hand from the slider. The operation direction of the slider is substantially perpendicular to the disengaging direction of the locking spring. Thus, in operating the slider, the locking section of the operating member moves away from the engaging portion of the locking spring and hence the engaging portion is capable of disengaging from the engaging notch. In this manner, the blade can be moved forward or backward in the slide groove.
According to this construction, the force applied from the blade to the slider during the use of the cutter knife is supported by the locking section . Therefore, the locking mechanism is capable of withstanding a great force on condition that the engaging portion of the locking spring is rigid. But it is necessary for the locking mechanism to have the second spring for regulating the position of the operating member when the operator's hand is released from the slider. Thus, the locking mechanism has a complicated construction.